Mechanochemical synthesis and characterisation of two new bismuth metal organic frameworks †

5560 | CrystEngComm, 2014, 16, 5560–5565 This journal is © The R BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, 12489 Berlin, Germany. E-mail: franziska.emmerling@bam.de; Fax: +49 30 8104 1139; Tel: +49 30 8104 1133 † Electronic supplementary information (ESI) available. CCDC-978120 contains the supplementary crystallographic data for [Bi(pydc)(NO3)2(H2O)2]·H2O (2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3ce42633e Cite this: CrystEngComm, 2014, 16, 5560


Introduction
5][6][7][8] Metal organic compounds consist of different tunable molecular structures and have accordingly a large variety of possible architectures and topologies.][11] In this context, MOFs containing bismuth as the metal centre are currently investigated in greater detail. 12Connected via organic ligands, bismuth cations offer a wide range of structural diversity.Bi 3+ cations are known for their stereoactive lone pair leading to interesting and versatile coordination geometries. 13Moreover, bismuth shows low toxicity even though its position in the periodic table suggests contrary findings.The pharmaceutical value of bismuth and its complexes are well documented, e.g.bismuth subsalicylate complexes have been used successfully for the treatment of gastritis and similar stomach diseases. 14,15icarboxylate ligands are widely used to link metal centres in metal organic frameworks.In combination with Bi 3+ cations as inorganic metal centres, the structural diversity is even higher.7][18][19][20][21] Small changes in the synthesis of these compounds can lead to different crystalline structures.Thirumurugan et al. reported four different MOF structures including Bi 3+ cations and benzene-1,4-dicarboxylate synthesised by changing the metal precursor or the reducing agent. 12Recently, Wibowo et al. presented solvothermal and hydrothermal syntheses of three different bismuth-pyridine-2,5-dicarboxylate structures by varying the applied base from potassium hydroxide to sodium hydroxide. 16These syntheses require a certain amount of solvents and reaction times up to several days.3][24][25][26][27] Mechanochemistry prevents the use of large amounts of solvents and proceeds at ambient temperatures.Typically, pure products are obtained in high yields within minutes.
In the present work, two metal organic compounds containing Bi 3+ cations as metal centres and benzene-1,4-dicarboxylate 2)) as ligands are presented.Both compounds were synthesised mechanochemically.Compound (1) is the solvent free analogue to the dma containing MOF reported by Thirumurugan et al.The crystal structure of (2) was solved and refined from powder diffraction data.The characterisation of the crystal structures was carried out by Raman spectroscopy and EXAFS measurements.

Results and discussion
Two metal carboxylates including Bi 3+ cations and benzene-1,4-dicarboxylate have been synthesised mechanochemically.For both syntheses, bismuth(III) nitrate pentahydrate was used as the precursor.Compound (1) was synthesised by grinding bismuth(III) nitrate pentahydrate together with the ligand benzene-1,4-dicarboxylic acid and imidazole as the proton acceptor in the ratio 1 : 2 : 4 (Scheme 1).Under mechanochemical conditions, bismuth nitrate decomposes and HNO 3 evaporates from the reaction mixture after opening the grinding jar.The completeness of the reaction was confirmed by powder X-ray diffraction measurements (Fig. 1

, top).
A comparison with database entries revealed a good agreement of the XRD pattern of (1) with the diffraction pattern of (dma)[Bi(1,4-bdc) 2 ] (dma = dimethylammonium) (Fig. 1, top). 12t is reasonable that the protonated imidazole cations replace the dma cations in the structure obtained under mechanochemical conditions leading to an isotypic structure.
For the synthesis of compound ( 2) bismuth(III) nitrate pentahydrate, pyridine-2,5-dicarboxylic acid and a small amount of sodium hydroxide solution (200 μL, 1 M) were ground together for 15 minutes.Sodium hydroxide acts as the reducing agent during the reaction.
The comparison of the powder diffraction patterns of bismuth(III) nitrate pentahydrate, pyridine-2,5-dicarboxylic acid and the product (2) indicates the completeness of the reaction (see Fig. 1, bottom).The diffraction pattern of (2) could not be assigned to any database entry.The structure was solved from the powder diffraction pattern leading to the first description of compound [Bi(pydc) The mechanochemically synthesised three-dimensional metal organic framework (H 2 Im)[Bi(1,4-bdc) 2 ] (1) crystallises in the rhombohedral space group R-3c.The crystal data of this structure are summarised in Table 1.The asymmetric unit cell consists of two different Bi 3+ cations, one benzene-1,4-dicarboxylate anion, and one imidazole cation.The two Bi 3+ cations Bi1 and Bi2 are present in the ratio of 1(Bi1) to 2(Bi2).Both are holodirected with a stereochemically inactive lone pair.Bi1 is twelve-coordinated and Bi2 is nine-coordinated by six different benzene-1,4-dicarboxylate molecules.The connection between the benzene-1,4-dicarboxylate molecules and the bismuth polyhedra results in a three-dimensional framework (Fig. 2). 1,3,12XAFS measurements at the Bi L 3 -edge were performed for compound (1).These measurements provide information about the coordination environment of the Bi 3+ cations.EXAFS data of structure (1) shown in real space are given in Fig. 3, left and the corresponding fit parameters are summarized in Table 2.The fitted comparison data were calculated based on the crystal structure.For the evaluation, single-scattering paths of the first coordination shell of the Bi centres were used.A good agreement of the experimental data with the fit results could be proven.The root mean square error (RMSE) of the atom distances of structure (H 2 Im)[Bi(1,4-bdc) 2 ] (1) is 0.145 Å.This proves a good agreement of the theoretical atom distances with the experimentally obtained distances.Thus, the crystal structure and especially the coordination spheres of the two Bi 3+ cations can be proven by EXAFS measurements.Structure of [Bi(pydc) The crystal structure of [Bi(pydc) 2) was calculated based on its powder diffraction data.Fig. 4 shows the result of the Rietveld refinement illustrating the good agreement of the simulated powder pattern with the measured one.The refinement converged at R wp = 10.4%.The crystal data are given in Table 1.[Bi(pydc)     The asymmetric unit cell contains one Bi 3+ cation, one pyridine-2,5-dicarboxylate anion, two nitrate anions and three water molecules.The Bi 3+ cation is nine-coordinated and holodirected with a stereochemically inactive lone pair of electrons.The Bi 3+ cation coordinates bidentate to the nitrogen atom and one carboxylate oxygen atom of the pyridine-2,5-dicarboxylate molecule.The coordination environment is completed by two water molecules, two nitrate units coordinating bidentate, and a third nitrate anion coordinating monodentate.In addition, one uncoordinated water molecule is near the second carboxylate unit of pyridine-2,5-dicarboxylate.The Bi-N bond length is 2.533 Å and the Bi-O bond lengths are in the range of 2.337-2.798Å.The monodentate coordinated nitrate anion connects this complex to the adjacent Bi 3+ cations.Consequently, a one-dimensional chain of ninecoordinated Bi 3+ cations is formed (Fig. 5).The chains are arranged parallel to each other running along the a-axis.
The coordination sphere around the Bi 3+ cation and thereby the crystal structure of compound (2) could be supported by EXAFS measurements conducted at the Bi L 3 -edge.EXAFS data shown in real space and the fit parameters are given in Fig. 4, right and Table 2.The fitted comparison data, based on the crystal structure from the powder diffraction pattern, showed a good agreement with the experimental data.The first scattering path was used for evaluation.The RMSE of the atom distances of structure [Bi(pydc) This proves a good agreement of the calculated atom distances with the experimental distances.
0][31][32] The Bi-O distances in these structures range between 2.21 and 3.This is © The Royal Society of Chemistry 2014 benzene-1,4-dicarboxylic acid and pyridine-2,5-dicarboxylic acid but they are clearly shifted in the products due to deprotonation and new coordination spheres of the ligands in the final products.For the synthesis of compound (1) the protonation of the added imidazole molecule can be verified based on the Raman spectra.Two intensive bands at 1220 cm −1 and 1445 cm −1 characterize the spectrum of the imidazolium ion (H 2 Im). 33These bands are clearly detectable in the Raman spectrum of compound (1).

Methods
Powder diffraction measurements.Powder diffraction measurements were performed using a D8 Discover diffractometer (Bruker AXS, Karlsruhe, Germany) equipped with a Lynxeye detector and operated in transmission geometry (Cu-Kα 1 radiation, λ = 0.154056 nm).Samples were measured over a 2θ range of 5-80°with a step size of 0.009°a nd 0.5 to 2.5 s per step.
Structure determination.For the high resolution X-ray powder diffraction experiments, the sample was sealed in a glass capillary of 0.5 mm diameter (WJM-Glas, Müller GmbH, Berlin, Germany).Powder diffraction data were collected at room temperature.Indexing of the powder pattern of (2) using the indexing module of TOPAS 34 led to a primitive triclinic unit cell with lattice parameters given in Table 1.The number of formula units per unit cell was determined to be Z = 1 from packing considerations, indicating P1 as the most probable space group, which could later be confirmed by Rietveld refinement.The unit cell and space group were confirmed using CHEKCELL. 35The structure determination of [Bi(pydc) 2) was carried out based on the powder XRD pattern.A structural starting model of Rietveld refinement was subsequently found using the Monte Carlo simulated annealing programme FOX. 36This programme uses global-optimization algorithms to solve the structure by performing trials in direct space.This search algorithm uses random sampling coupled with simulated temperature annealing to locate the global minimum of the figure-of-merit factor.Parts of the molecule were refined as a rigid group to reduce the total number of degrees of freedom.Both nitrate molecules were set rigid, while pyridine-2,5-dicarboxylate was set rigid with the exception of the carboxyl oxygen and hydrogen atoms.In addition, a bond between the Bi atom and the nitrogen and one carboxyl oxygen atom of pyridine-2,5-dicarboxylate was assigned.The crystal structure of [Bi(pydc) 2) was solved by the simulated annealing procedure.The calculation process was carried out using a standard personal computer within 20 hours, finding the deepest minimum of the cost function several times during the procedure.The subsequent Rietveld refinement was performed using the TOPAS software. 34The structural solution obtained from MC/SA was subsequently subjected to a Rietveld refinement.The refinement converged at R wp = 10.4 %.Selected bond lengths and angles of compound (2) are given in the ESI † (S-Table 1 and S-Table 2).
Raman spectroscopy.The Raman spectra were collected using a Raman RXN1™ analyzer (Kaiser Optical Systems, Inc., Ecully, France) with NIR excitation radiation at 785 nm.The spectrometer is equipped with a CCD camera (1024 × 256 pixels) and a non-contact probe head (working distance 15 mm, spot size 1 mm Ø).The Raman spectra were recorded with an acquisition time of 5 × 5 s and an irradiance of 6.6 W cm −2 on the sample.
EXAFS measurements.For the EXAFS measurements, both products were mixed with activated carbon and fixed in plastic sample holders.EXAFS measurements were performed at the electron storage ring BESSY II (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany) at the BAMline. 37,38The X-ray beam was monochromatised using a double-crystal monochromator (DCM).The size of the beam spot was 3 × 1 mm 2 .The energy resolution of the setup was about 0.00015, leading to ΔE = 2.0 eV while measuring at the Bi L 3 edge (13.419 keV).The excitation energy was varied from 13.250 to 14.271 keV in steps of 10 eV pre-edge, 1 eV at the edge and Δk = 0.04 in the k-space of the EXAFS region.Measurements were performed in transmission geometry.The first ionisation chamber was filled with argon (1 bar) and the second was filled with xenon (1 bar).The EXAFS data were evaluated using the programmes Athena, Artemis, and Hephaestus. 39The model spectra of the fits were calculated using the FEFF9 code. 40lemental analysis.To exclude the presence of X-ray amorphous impurities, elemental analyses were conducted.The results are in good agreement with the expected values.Compound (1): BiC 19 H 13 O 8 N 2 (606.30g mol −1 ): calculated: C 37.64%, H 2.16%, N 4.62%; found: C 32.05%, H 3.10%, N 4.85%.Compound (2): BiC 7 H 9 N 3 O 13 (552.13g mol −1 ): calculated: C 15.23%, H 1.64%, N 7.61%; found: C 14.38%, H 1.73%, N 7.78%.

Conclusions
Two metal organic compounds containing bismuth and benzene-1,4-dicarboxylate or pyridine-2,5-dicarboxylate as ligand were synthesised mechanochemically.The crystal structure of compound (1) was identified based on its powder diffraction data.The crystal structure of the bismuth and pyridine-2,5-dicarboxylate containing compound (2) could be calculated from its powder diffraction pattern.The mechanochemical synthesis pathway revealed a fast, efficient, and solvent free access to both compounds.For both structures, Raman spectra and EXAFS measurements revealed a good agreement of the calculated crystalline structures and the bismuth coordination spheres.

Fig. 4
Fig.4Scattered X-ray intensity of structure (2) under ambient conditions as a function of diffraction angle 2θ.The observed pattern (circles), the best Rietveld fit profile (red line), the reflection positions (blue tick marks), and the difference curve (grey line) between observed and calculated profiles are shown.The wavelength is λ = 1.54056Å (Cu-Kα 1 ).The R-values are R p = 8.0%, R wp = 10.4%; for R p and R wp , refer to the Rietveld criteria of fit for profile, weighted profile, and structure factor, as defined by Langford and Louer.28

Fig. 5
Fig. 5 View of the chain structure [Bi(pydc)(NO 3 ) 2 (H 2 O) 2 ]•H 2 O (2) along the b-axis (top).Representation of the packing of the molecules in a projection along the a-axis (bottom).For each projection, the polyhedron around one Bi 3+ cation is shown in violet.The hydrogen atoms of the water molecules are omitted for clarity.